1. Field of the Invention
The present invention relates to an image forming apparatus which is usable as a electrophotographic copier or printer and, more specifically, it relates to a toner supply or replenishing device and method in such an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus of two-component developing type which uses a developing agent containing toner and carrier, it is very important to maintain a constant mixture ratio of the toner and the carrier for maintaining the desired level of image quality. In order to maintain the mixture ratio of the toner and the carrier (the mixture ration being referred to as toner density hereinafter) constant, it is necessary to detect the toner density of the developing agent and to supply a toner replenishing device with toner of the same amount as consumed. As a method of detecting the toner density, directly reading the toner density of the developing agent with a magnetic sensor is widely employed.
On the other hand, in an image forming apparatus of two-component developing type wherein a plurality of developing devices is disposed in a rotary revolver unit for successive development of respective colors, such as digital full-color copier, because the developing devices are mounted in the revolver unit which is a rotating body, mounting magnetic sensors to the respective developing devices for detecting the toner density requires a very complicated and costly interface for supplying power to the magnetic sensors and transmitting output information from the magnetic sensors to an electrical device of the apparatus.
In order to solve this problem, Japanese Patent Application KOKAI Publication No. 2001-34018 proposes a method according to which a given toner image is applied or formed on a photosensitive drum or a transfer belt, toner density of the toner image is detected by means of a photo sensor, the toner density of the developing agent is calculated from the density data obtained by the detection, and replenishing of the require amount of toner is performed accordingly. However, this method requires additional processes of toner development to form a latent image on the photosensitive drum and toner image transferring to the transfer belt, so that the toner density detection results are susceptible to external influences such as environment, the photosensitive drum and deterioration of the developing agent, with the result that stabilized detection results cannot be obtained in many cases. For reducing such external influences, it has been necessary to provide a very complicated control to compensate for the toner density on the basis of information from temperature and humidity sensors for the environment, sensor for the photosensitive drum potential and life counter for the developing agent.
According to Japanese Patent Application KOKAI Publication No. Hei 5-27528, there is provided a table showing the relationship between the toner replenishing time and toner replenishing amount for each color toner, the output level of digital image signal of each color is counted for each pixel, the video count number for each color is calculated, predictably determining the toner replenishing time for the image on one original corresponding to the video count number for each color inputted with reference to the above table and toner replenishing is performed accordingly. In this prior art, however, since the toner replenishment is performed on the basis of prediction, there is a fear that an error occurs in the toner replenishing amount due to variation from a set value of toner replenishing mechanism, etc. and, therefore, further additional density control mechanism is required to avoid the error, thus making the apparatus costly and complicated.
It has been proposed by still another method to calculate the density of developing toner by reading directly the reflectivity of light from the developing agent layer on the development sleeve of the developing device by means of a photo sensor with the revolver unit located at the standby position. As compared with the above-described methods, this method does not require additional developing process or transferring process, thus making possible stabilized detection of toner density.
According to this method, however, since the density detection is not performed at the developing position or printing position where the toner is actually consumed and replenished, but at the standby position of the revolver unit, toner replenishment according to the detection result is performed only for the next developing process. For this reason, in printing from an original having a high coverage rate, toner density is reduced below a predetermined range, with the result that problems occur such as reduced image density and attachment of carrier to the photosensitive drum.
Therefore, it is an object of the present invention to provide a toner replenishing device of an image forming apparatus which is simple in structure, solves the disadvantages inherent in detecting the density of toner on the development sleeve at the standby position of the revolver and makes possible stabilized toner density controlling irrespective of whether the coverage rate of the original rate is low or high, as well as a toner replenishing method and an image forming apparatus using such device and method.
In order to achieve the object, there is provided a toner replenishing device in an image forming apparatus having a plurality of developing devices, wherein the developing devices are changeable between the developing position and standby position and electrostatic latent image on a photosensitive body is developed, which replenishing device comprising a toner density detector for detecting toner density of developing agent layer of the developing device located at the developing position, an original coverage rate calculator for calculating the coverage rate of an original on the basis of data of image printed by the developing device at the developing position, and a control for executing replenishment of a required amount of toner to the developing device at the developing position on the basis of the detected toner density when the developing device is changed from the standby position to the developing position if the coverage rate of the original is a predetermined valve or less than that or less than the predetermined value, and executing replenishment of a required amount of toner to the developing device located at the developing position for printing if the coverage rate of the original is a predetermined valve or higher than that or higher than the predetermined value.
In such a device, calculation of the original coverage rate by the original coverage rate calculator may be performed on the basis of data of image which is being developed by the developing device during the development by the developing device. Furthermore, the original coverage rate calculator is operable to calculate the coverage rate of the original on the basis of data of image printed from the moment of print starting from the top end of the image to be printed until a predetermined printing width of image has been printed, and this predetermined printing width may be changeably set. Additionally, the predetermined value of the coverage rate of the original may be provided as progressive values or continuous values. The toner density detector may be constructed so as to be comprised of a photo sensor for detecting the light reflectivity of the developing agent layer and a toner density calculator for calculating the toner density on the basis of the detected light reflectivity.
An image forming apparatus of the present invention is constructed comprising a plurality of developing devices which are changeable between developing position and standby position, a photosensitive body on which an electrostatic latent image is developed into a toner image by the developing device located at the developing position, an exposure unit for exposing the photosensitive body to light for forming the electrostatic latent image on the photosensitive body, a toner density detector for detecting toner density of developing agent layer of the developing device located at the developing position, an original coverage rate calculator for calculating the coverage rate on the basis of data of image formed by exposure to light on the photosensitive body; and a toner replenisher for replenishing toner to the developing device on the basis of the original coverage rate and the toner density.
In such an image forming apparatus, the original coverage rate calculator is operable to calculate the original coverage rate by adding up the data of image formed by exposure to light on the photosensitive body simultaneously with the development by the developing device at the developing position.
Still furthermore, the present invention provides a toner replenishing method in an image forming apparatus having a plurality of developing devices, wherein the developing devices are changeable between the developing position and standby position and electrostatic latent image on a photosensitive body is developed, comprising the steps of calculating toner density of developing agent layer of the developing device located at the developing position, calculating the coverage rate of an original on the basis of data of image printed by the developing device at the developing position, and replenishing a required amount of toner to the developing device at the developing position on the basis of the calculated toner density when the developing device is changed from the standby position to the developing position if the coverage rate of the original is a predetermined valve or less than that or less than the predetermined value, and replenishing a required amount of toner to the developing device located at the developing position for printing if the coverage rate of the original is a predetermined valve or higher than that or higher than the predetermined value.
In such a toner supply or replenishing method, if the coverage rate of the original is a predetermined valve or higher than that or higher than the predetermined value, toner replenishing to the developing device at the developing position may be initiated before the developing device is changed to the standby position where the toner density detection is performed.
According to the above described structure, the toner density will not be reduced to a range where faulty image may be produced, and faulty image will not formed neither at the bottom end portion of a paper sheets in the first printing nor at the top end portion of a paper sheet in the second printing. Thus, the present invention can solve the disadvantages inherent in detecting the density of toner on the development sleeve at the standby position of the revolver and makes possible stabilized toner density controlling irrespective of whether the coverage rate of the original rate is low or high by using a simple structure.